1. Field of the Invention
The present invention relates to transmissions for motor drive shafts; and in particular to variable ratio transmission elements for rotating drive shafts, including transmission elements that use electroactive polymer devices.
2. Description of the Related Art
Conventional transmissions for electric motors and combustion engines are widely used in commerce but are not appropriate for all applications. For example, there is use for a class of balloon vehicles that supports a payload at high altitude at a relatively fixed geographic location for extended periods of time. The high altitudes may include the stratosphere layer of the atmosphere or the atmosphere of a different planet, such as Mars. Such a vehicle can support a communication terminal or surveillance system that covers a much wider area than can be served by a tower and gives more temporal coverage and detail than can be achieved from a winged aircraft or an orbiting satellite. To maintain geographic position in the presence of high altitude winds, where the air has low density, a motor and transmission are useful that can turn a station-keeping propeller that is both large and slowly rotating. Another class of vehicles includes spacecraft and satellites, which need light, efficient technology, for example to extend arms and panels or open or close compartments. Rovers delivered by these vehicles—like the Mars rovers—also could use lightweight and efficient motors and transmission elements.
A desirable motor and transmission would be capable of providing one to several horsepower. For extended duration, the motor and transmission should be highly efficient, turning most of the energy available on board into mechanical rotation and dissipating very little as heat. To leave capacity for payload, the motor and transmission should be light, e.g., on the order of fifty pounds or less. In applications that benefit from security against attack from hostile entities, the whole platform, including the motor and transmission, should have small radar reflectivity, e.g., be as transparent to radar as plastic materials.
Transmissions for conventional engines include gear boxes and metal gears that are both heavy and highly reflective to radar signals. In some vehicles, a small electric or compressed gas or EAP motor may be used with a mechanical transmission system to covert high rpm, low torque motors to low rpm, high torque power sources. However, a metallic gearbox mechanical transmission system suffers from the same deficiencies of heavy weight and large radar reflectivity that a larger motor would.
A new class of devices uses electroactive polymers (EAPs) to convert from electrical to mechanical energy. When a voltage is applied to electrodes in contact with an EAP, the EAP deforms. An EAP sandwiched between stretchable electrodes deforms in two dimensions as described in U.S. Pat. No. 6,664,718 by Pelrine et al., entitled “Monolithic Electroactive Polymers” (hereinafter, Pelrine). Metallic content can be reduced substantially in such devices. The electrodes can be thin or formed with non-metallic conductors, such a carbon nanotubes. The transfer from electrical energy to mechanical energy is quite efficient.
Many prior art transmissions require multiple wheels to provide multiple rotation rate ratios. The multiplicity of wheels in a gearbox leads to larger size, greater weight, more complexity, and less reliability, than a transmission that can achieve variable rotation rate ratios using a single axle.
Based on the foregoing there is a clear need for a mechanical transmission system that does not suffer the disadvantages of prior art mechanical transmission systems. In particular, there is a need for a lightweight, low-metallic mechanical transmission system with variable rotation rate ratios using a single axle.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.